n .One of the key issues for treatment of central nervous system (CMS)diseases is finding effective means to deliver drugs through the blood-brain barrier (BBB). This block is even more difficult to circumvent when delivering large macromolecules such as neurotrophic factors, nucleic acid based therapeutic reagentsor gene therapy vectors that encode such factors. The first step towards effective delivery of therapeutic genes to the CMSwould be the development of gene therapy vectors that target the CMS specifically. Ideally, these vectors could be injected directly into the bloodstream and home to the blood-brain barrier, transduce the capillary endothelial cells and either express their therapeutic genes within the endothelial cells or following delivery into the brain. Towards this end, we made significant progress in developing lentiviral targeting vectors for stable introduction of genes into specified cells and tissues. The central hypothesis of this proposal is that efficient targeting vectors with high selectivity for the CNS can be developed to deliver therapeutic genes into the CNS. We recently reported that a lentiviral vector pseudotypedwith a modified Sindbis virus envelope bearing the Fc binding domain of protein A can be retargeted based upon the specificity of monoclonal antibodies directed to specific cell surface molecules. As proof of principle, we demonstrated that this vector can be injected intravenously into the mouse tail vein and specifically transduced metastatic melanoma cells bearing P-glycoprotein as the target molecule (Morizono K, et. al,. 2005 Lentiviral vector retargeting to P- Glycoprotein on metastatic melanoma via intravenous injection. Nat Med. 11(3):346-52.). Because the approach can be generalized to other ligand/antibody combinations we propose here to develop lentiviral gene therapy vectors that will specifically target the blood-brain barrier. In that regard, our preliminary studies indicate that further modification of the targeting vector and conjugation with anti-transferrin receptor (TfR) antibodies can result in targeting through the bloodstream to what appears to be specifically the CNS. Given our long-standing interest in retroviral biology, recently applied to gene therapy vectors, and the novel targeting approach described here, we feel that we are well positioned to address the questions posed in this application.